Early-exit validation: stop once enough valid proxies found

- validate_proxies: target= param — once N valid found, remaining
  tasks skip the HTTP call and drain immediately (no wasted time)
- _build_pool: target = max(min_pool_size, chain_length * 3)
- Settings: min_pool_size (default 20), max_candidates default 400->200
- UI: Stop after N valid / Max candidates fields in Settings tab
- README: updated settings table
- 1 new test for early-exit with unreachable target

Made-with: Cursor

proxy_chain_manager/app.py

@@ -759,8 +759,10 @@ def main() -> None:
 
     val_sec = _section("Validation")
     _fld(val_sec, "Concurrent validators", "conc", str(s.validation_concurrency))
-    _fld(val_sec, "Max candidates per cycle", "maxc", str(s.max_candidates),
-         "(random sample from fetch)")
+    _fld(val_sec, "Stop after N valid proxies found", "minpool", str(s.min_pool_size),
+         "(early-exit — stop testing once enough found)")
+    _fld(val_sec, "Max candidates to test per cycle", "maxc", str(s.max_candidates),
+         "(shuffled random sample from fetch)")
     _fld(val_sec, "Per-proxy timeout (sec)", "timeout", str(s.validation_timeout_seconds))
 
     sec_sec = _section("Security")
@@ -808,6 +810,7 @@ def main() -> None:
                 health_check_seconds=min(3600, max(10, int(entries["health"].get().strip()))),
                 full_refresh_seconds=min(86400, max(60, int(entries["refresh"].get().strip()))),
                 validation_concurrency=max(1, int(entries["conc"].get().strip())),
+                min_pool_size=max(3, int(entries["minpool"].get().strip())),
                 max_candidates=max(10, int(entries["maxc"].get().strip())),
                 validation_timeout_seconds=min(120.0, max(2.0, float(entries["timeout"].get().strip()))),
                 prefer_elite=bool(elite_var.get()),